Source : Webster's Revised Unabridged Dictionary (1913)

Erst \Erst\ ([~e]rst), adv. [Orig. superlative of ere; AS.
   [=ae]rest. See {Ere}.] [Archaic]
   1. First. --Chaucer.

   2. Previously; before; formerly; heretofore. --Chaucer.

            Tityrus, with whose style he had erst disclaimed all
            ambition to match his pastoral pipe.  --A. W. Ward.

   {At erst}, at first; at the beginning.

   {Now at erst}, at this present time. --Chaucer.

Source : WordNet®

erst
     adv : at a previous time; "once he loved her"; "her erstwhile
           writing" [syn: {once}, {formerly}, {at one time}, {erstwhile}]